A half-imported demo is the most common reason a theme gets a one-star review, and almost always the cause is environmental rather than the theme itself.
The three usual causes
A PHP memory or execution limit that kills the import mid-run, a required plugin that was not active when the import started, and a media step that timed out fetching images over a slow connection.
Recovering without starting over
A good importer is idempotent — running it again should top up what is missing rather than duplicating what arrived. If yours is not, import a single page instead of the whole demo and rebuild the gaps.
Preventing it next time
Raise the memory limit before you start, activate the required plugins first, and run the import on a connection you are not sharing with a video call.